1. Field of the Invention
This invention relates to a device for detecting the residual capacity of a battery.
2. Description of the Related Art
A number of battery-operated electronic device are designed so that they produce an alarm when the residual capacity of their battery falls below a predetermined level during use. More specifically, the terminal voltage of the battery is monitored and compared with a reference voltage. When the terminal voltage is found to be below the reference voltage, an alarm is given, for example, as a sound, by an indicator lamp illuminating, or by an indicator blinking on a display screen.
However, as is shown in FIG. 1, the terminal voltage of a battery varies widely with the varying magnitude of a discharge current (variations in load). Thus, in the case of equipment in which large current variations (load variations) occur, when the discharge current is small an alarm will not given without extreme lowering of the residual capacity of battery. Where the discharge current is large, on the other hand, an alarm will be given even when the battery has sufficient residual capacity. Accordingly, by comparison between the terminal voltage of battery and the reference voltage alone, it is impossible to accurately detect the residual capacity of the battery.
U.S. Pat. No. 4,587,640, issued to Yuichi Saitoh and assigned to the same assignee as this application, discloses a cassette-type semiconductor memory device which comprises a RAM (random access memory) and a backup battery, and a voltage monitoring circuit, and can be connected to a data processor by way of a built-in connector. When the cassette is inserted into the connector of a processor the power supply of which is turned ON, a voltage is generated which is monitored. Upon detection of a voltage other than a normal voltage, access to the RAM is prohibited and a standby condition is set. When detecting that the voltage on a power line from the data processor has been raised to a normal value, the monitoring circuit issues a permission signal enabling access to the RAM to be resumed. With the prior art, however, variations in current dissipation on the power line are not taken into consideration.
The U.S. patent application titled "A Power Supply With A Built-In Microcomputer For Use In A Computer System" filed Dec. 17, 1987 by Takuma Yamasaki and Yuichi Saitoh, and bearing Ser. No. (unknown) discloses a power supply for a computer system. This power supply contains a microcomputer which confirms a saving operation of a computer system by communicating with the computer system and interrupting the power supply after confirmation.
In the application, however, no technique for varying a detection level of the residual capacity of a battery in accordance with the magnitude of a discharge current is disclosed.